Gas filled dosimeter and method of manufacturing the same



July 19, 1960 o. G. LANDSVERK ETAL 2,945,734

GAS FILLED DOSIMETER AND METHOD OF' MANUFACTURING THE SAME Filed Aug. l2, 1958 Frs. j.

01.5 6. Ayom/RK 05x. afer L. Mime/5R 1NvENToRs GAS FILLED DOSllVIETER AND METHOD F MANUFACTURING THE SAME Ole G. Landsverk and Delbert L. Merriner, Glendale, Calif., assignors to Landsverk Electrometer Company, Glendale, Calif., a corporation of California Filed Ang. 12, 1958, Ser. No'. 754,599 -3 Claims. (Cl. 316-20) This invention pertains to new and improved gaslled nited States Patent ,ICC

' inert gases asherein specied the sensitivity of these indosimeters and to a method of manufacturing the same.

The term dosimeters is commonly used in several dilerent manners. Frequently this term is used only to designate quartz `fiber devices for measuring exposure to radioactive radiation. A known type of a quartz fiber dosimeter is shown in the co-pendingV U.Sv. patent application, Serial No. 645,059, dated March 11, 1957. The term dosimeter is however, capablevof broader usage, and may be usedV to designate devices which are more properly known as pocket chambers androentgen meters. A difference between these last twotypes of instruments and quartz iiber dosimeters Vlies in the fact that the latter two instruments are not direct reading.

Pocket meters and roentgen meters are. formed so as to include an electrode projecting into an ionization chamber. Such an ionization chamber is also used in a quartz `liber dosimeter. Within these instruments a quartz fiber' voltmeter is mounted within the ionization chamber.`

The ionization chambers -withinboth of. the .types of instruments indicated have conventionally been vfilled with air. This has been discovered to be -disadvantageous because of the effect of air.on the performance of theseV instruments. Air contains oxygen, a rather reactive gas. When anv ionization chamber isherrnetically-sealed so as to contain airV the oxygemcontent of the .air will gradually` react with materials in contact with it. As a Vresultl the sensitivity of these known instruments -will gradually deV struments remains relatively constant throughout their effective life. v f

,Because of the nature of this invention, it is not considered necessary to set forth in this specification a further list of` various objects and advantages of the invention itself. Various iother objects and advantages of the invention'will be fully apparent to those skilled in the art to `which the invention pertains from'a detailed consideration ofthe remainder of this description, including the appended `claims and the accompanying drawing in which'. i g

Fig. 1 is a cross-'sectional View of a quartz ber dosimeter' manufactured in accordance with this invention;

Fig. `2 is `a similar View of a pocket chamber'dosimeter,

manufacturedA in accordance with'this invention.

the "inherent-character of the Ainvention as brieflyy indicated abovegthose skilled in the'art Ito which this invent-ion pertains will realize that the invention may be vused of` about 1% per month.. The. rate of decrease of sensitivity will, of course, vary with temperature. At a temperature of about :150 F. a common quartz ber dosimeter `will decrease `about 10% per month in sensitivity. These, decreases in sensitivity `are,"however, limited by the amount of oxygen in the air available Vto react with various `materials within aydosimetfer.

The sensitivity of `any dosimeter will vary with a number of different factorsfsuch as, for example, the internal dimensionsfof the instrument andthe like. ABecause of these and a number of other factors itis diicult tomandfacture dosimeter-s `so that their sensitivities are uniform whenthey `areltlled with air at the same pressure. The commercial specifications fordosimeters require that their sensitivities fall within a comparativelynarrow range.

In order to correct the variations in the sensitivity of these instruments several different expedients have been adopted. One of these is tocontrolthe pressure of the air within a hermetically sealed instrument prior to its being' sealed by sealing itV in a vacuum or a pressure chamber. Another method has been to put dosimeter's Vhaving low sensitivity in a refrigeratorfand to seal them when they are cold, or to put dosimeters having too high a sensitivity in an oven and to sealth'em while they are at an elevated temperature. It has also been known to transport` dosimeters beforesealing `them to analtitude in which the airpressureis such that the sensitivity of .a1

with virtually any type of dosimeter or roentgen meter employing an ionization chamber. i

As 'an aid to understanding thisinvention it may be stated'in essentially summary form that it concerns filling the ionizationchamber within a dosimeter with an inert gas` which isincapable of reacting with various parts of the dosimeter 'in contact with this gas. Preferably the inert gas employed is a gas mixture having an effective atomic .number which is about the same as the effective atomic number :of air so that the sensitivity of the dosimeter Ymay be determined in air .and 'then this air maybe replaced with the *inert gas, and lthe instrument sealed.

' The inert gas mixture employed may, however, have an effective atomic number. which differs slightly from the effective atomic number of air soas to change the sensitivity .of the dosimeter if this is required.

t The .actualnature Yof this invention is best more fully explained by Ireferring directly to V.the accompanying drawing. In Fig.. lfthere is shown a quartz liber dosimeterV of the general type shown in the aforenoted co-pending U.S.V patent application, Serial No. 645,059. For conveniencefonly `the principal parts of this dosimeter 10 are specifically designated herein. This dosimeter 10 includes an elongatedrtubular housing 12 having a clipy 14. mounted thereon so that it may be carried in the pocket of an -individual or the like. A glass plate 16 is sealed to the housing 10 at the upper end of the dosimeter 10, so that an individual using it may look through a microscope assembly 18 Iand then lthrough a reticule 19 so as to view the position of a quartz liber 20 carried upon a metal frame 22.

The frame Z2 and the ber 20 are mounted within an ionization chamber24 which is preferably lilled as hereinafter described. The frame 2271's, as indicated, carnied upon a non-conductive spacer y2,6 and `directly con-x condenser-nis electrically connected to the housing k through a spring clip 32. A charging pin 34 is mountedV Patented July 19, 1960 38 which is sealed with respect to the housing 12. 'Ihe charging pin 34 is adapted ,to be moved so as to be in contact with a metal cross member 40 when the complete dosimeter is being charged. The cross-member iits against the tube 28 so as lto be in electrical contact with this tube. Y Y

From this brief description of the dosimeter 10 it will be seen that the entire instrument is hermetically sealed lat its ends, and that within this instrument there are located a number of different parts formed out of a variety of different materials. Many of these parts are formed out of various thermoplastic resinous materials. Because of the nature of the various materials within the housing 12 it is possible for any air within this housing and within the ionization chamber 24 lto gradually react so that the oxygen content of the air within .the dosimeter 10 is slowly lowered, lowering the sensitivity of the complete dosimeter `10 as previously indicated in the initial par-t of the specification. l In order to remedy this difculty in accordance with the broad teachings of this invention, `it is possible to iill the ionization chamber 24 and the complete dosimeter 10 with an appropriate inert gas. The term ine is not used herein an absolute sense but is intended to desig-v nate a gas which is inert with respect to the materials within the dosimeter 10 .under the temperature conditions normally encountered. Such inert gases `are well known. Various gases of Group I of the Periodic Table, such as helium, argon, neon and the like can be employed, since they are chemically inert under virtually all conditions. Qther gases such as nitrogen and the like can be also satisfactorily employed. y Y The sensitivity ofthe dosimeter 10 will, of course, vary depending upon the naturevof the specific gas or gas mixture located within the ionization chamber 24. For this reason it is preferred with the present invention to IIt has been found that a mixture of about 7 parts by volume of nitrogen and about 1 part by volume of argon will give approximately the same sensitivity as air when used within a dosimeter such as the dosimeter 10. In carrying `out this invention the air within the dosimeter 10 may be evacuated through a small hole 42 in the side wall, and then the instrument may be filled with a gas mixture of this type, and after this gas mixture has reached atmospheric pressure the hole may be sealed in accordance with known teachings, as by the use of solder. Various other similar known techniques for placing a desired gas mixture within the interior of the dosimeter 10 may also be employed. These techniques will obviously have to vary Vsomewhat depending upon the precise construction of any specic instrument.

With the preferred method of using this invention before the operations indicated in the preceding paragraph are carried out a dosimeter such as the dosimeter 10 is tested in air of :atmospheric pressure in order to determine its sensitivity and then this instrument is iilled with an inert gas mixture using the procedure indicated. The gas mixture used in lling the instrument is, however, may be varied depending upon the sensitivity of this instrument itself. If, because of various manufacas helium, xenon or the like can be readily created using utilize within this dosimeter a gas mixture which, as far l as the operation of the dosimeter is concerned, behaves the same as air; As a result of the use of such mixtures the dosimeter 10 can be tested and calibrated with air within the instrument in accordance with known practice, and then can be lled with an appropriate inert gas mixture and hermeticallysealed.

The operation of the dosimeter 10 is, of course, de pendent upon the formation of ions from the gas within the ionizationV chamber 24. These ions are largely caused by beta rays which emerge from the wall of the ionization chamber and hit this gas. The quantity of ionization carried by a given quantity of electrons is proportional yto the total number of electrons that the gas atoms within the ionization chamber contain within their shells. Because of the fact that all gases under the same temperature and pressure conditions have the same number of atoms within a given volume, the number of ions' that will be formed within the ionization chamber 2:4 is in proportion to .the atomic number of the gas or gases within the ionization chamber. I-t will be Yrealized that this latter represents a simplification of a complete discussion of the .formation of ionsY in a dosimeter such as the dosimeter 10, but that it is substantially correct, and that the effective atomic number of any gas mixture can be calculated by considering .the relative portions of gases within such a mixture. i

The eiective atomic vnumber of air is about 7.2. Helium, an inert gas, has an atomic number of 4; thus, when helium is used in .the ionizationV chamber 24 the dosimeter is roughly 55% as sensitive as when air is used.. Similarly, the` use of nitrogen alone in the ionization chamber 24k lowers the sensitivity of the dosimeter 10 because its atomicnumber is 7. Argon, having an atomic number of 18, produces more ionization trom a given number of .beta rays :than air within a dosimeter' and; hence, 'increases the sensitivity of an instrument of known techniques so as to provide a gas mixture having an effective atomic number which is sutiicient to either maintain the sensitivity of a dosimeter or to alter its sensitivity as may be required.

The broad features of this invention can be applied to a number of other instruments besides dosimeters. In Fig. 2 of the drawing there is shown a pocket chamber dosimeter or roentgen meter which is constructed so as to be similar to the dosimeter 10. For convenience those parts of the meter 50 which are identical or substantially identical with the dosimeter 10 aredesignated by the primes of the numerals previously used.

, In the instrument 50 the frame 22 and the quartz fiber v 24 are replaced by a collecting electrode 52 which extends into an ionization chamber 24. The end 54 of this collecting electrode 52 is formed so as to have a cup-like shape, and ts against a tube 28 so as to be in electrical contact therewith. The housing 1'2' above the chamber 24 is provided with a closed end 56 in the pocket chamber dosimeter 50. This chamber 24', and, in fact, the

' measuring exposure to radioactive radiation besides the specific devices 10 and 50 herein described. For this reason this invention is to be considered as being limited only by the appended claims.

We claim: Y

l. A method of manufacturing a dosimeter containing an ionization chamber so that said dosimeter has a predetermined sensitivity to radioactive radiation, said method comprising: measuring the sensitivity of said dosimeter with air at atmospheric pressure in said ionization chamber; replacing the air inV said chamber by a gas mixture having an effective atomic number suicient to cause the sensitivity of said dosimeter to have a desired value; and sealing said gas mixture within said ionization chamber.

2. A method of manufacturing a dosimeter containing an'ionization chamber so that said dosimeter has a premixture which is inert with respect to al1 parts of said 5 dosimeter in contact with said gas mixture and which has an effective atomic number sufficient to cause the sensitivity of said dosimeter to have a desired value; and sealing said gas mixture within said ionization chamber.

3. A method of manufacturing a dosimeter containing 10 an ionization chamber so that said dosimeter has a predetermined sensitivity to radioactive radiation, said method comprising: measuring the sensitivity of said dosimeter with air at atmospheric pressure in said ionization chamber; replacing the air in said chamber by a 15 b mixture of nitrogen and argon which is inert with respect to al1 parts of said dosirneter in contact with said mixture of nitrogen and argon and which has an eiective atomic number sufficient to cause the sensitivity of said dosimeter to have a desired value; and sealing said gas mixture within said ionization chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,215,441 Ruse t Sept. 17, 1940 2,574,820 Feuer Nov. 13, 1951 2,701,175 Mazzagatti Feb. l, 1955 2,731,568 Failla Jan. 17, 1956 2,805,345 Warmoltz et al. Sept. 3, 1957 

